Toward advanced high-speed wireless communication system, such as transmitting data in a higher peak data rate, the Long-Term Evolution Advanced (LTE-A) system is standardized by the 3rd Generation Partnership Project (3GPP) as an enhancement of the Long-Term Evolution (LTE) system. The LTE-A system targets faster switching between power states, improves performance at the cell edge, and includes subjects, such as bandwidth extension, coordinated multipoint transmission/reception (COMP), uplink multiple input multiple output (MIMO), etc.
For bandwidth extension, carrier aggregation (CA) is introduced to the LTE-A system, where two or more component carriers are aggregated for supporting wider transmission bandwidths and for spectrum aggregation. By implementing carrier aggregation, multiple component carriers are aggregated into overall wider bandwidth, where a user equipment (UE) or mobile device may establish multiple links corresponding to the multiple component carriers for simultaneously receiving and transmitting radio signals. In practice, the mobile device simultaneously operates in two or more frequency bands to achieve carrier aggregation.
Please refer to FIG. 1, which illustrates a spectrum of operating frequency bands utilized in the LTE-A system according to the prior art. As shown in FIG. 1, low frequency bands LB1-LB3 are ranged from 700 MHz to 900 MHz, a medium frequency band MB is ranged from 1800 MHz to 2100 MHz, and a high frequency band HB is ranged from 2300 MHz to 2600 MHz. The mobile device requires single frequency band if operating in a non-CA mode, and requires two or more frequency bands if operating in a CA mode such as modes CA1, CA2 and CA3 for different geographical areas and countries. For example, the frequency bands LB1 and MB are required to operate in the mode CA1, the frequency bands LB2 and MB are required to operate in the mode CA2, and the frequency bands LB3 and MB are required to operate in the mode CA3.
In addition, there is a trend of equipping the mobile device with a metal cover or a metal frame for industrial design and robustness, which may cause decreased antenna gain, narrowed bandwidth or unstable antenna performance due to the metal housing or frame when an antenna is integrated in the mobile device. In that situation, a designer not only faces a challenge of the antenna performance but also integration difficulty between the antenna and the metal cover.
To solve this issue, one of the conventional solutions is to distinctly design, antennas with separate stock keeping unit (SKU) for supporting the modes CA1, CA2 and CA3, which increases production cost and stock management efforts. Another conventional solution is to utilize a tunable antenna module including an antenna and a switch circuit in the mobile device, where the switch circuit is used for switching operating frequencies of the antenna to operate in the modes CA1-CA3 and the non-CA mode. However, the switch circuit causes second and/or third harmonic spur to interfere receiving signals of the antenna, in which the second and/or third harmonic spur of transmitting signals are reflected by the switch circuit such that the reflected signals are received by the antenna.
For example, in a case of one uplink with a carrier frequency (e.g., 704 MHz-716 MHz) and two downlinks in the medium frequency band MB (e.g., 1800 MHz to 2200 MHz) are established for the mode CA1, the second and/or third harmonic spur of the transmitting signals (e.g., 1408 MHz-2112 MHz) may interfere the receiving signals in the medium frequency band MB to reduce a signal-to-noise ratio of the receiving signals.
Therefore, how to improve the bandwidth and mitigate the harmonic interference to support carrier aggregation for the antenna integrated with the metal cover has become a goal in the industry.